Reactivation of herpes simplex virus (HSV) was assumed to be dependent on prostaglandin synthesis [Kurane et al. (1984) J Gen Virol 65: 1665-1674]. Since free radicals are generated in the course of prostaglandin synthesis and inhibitors of prostaglandin synthesis are free radical scavengers, the question arose whether free radicals are most important in viral reactivation. Therefore, the influence of five different inhibitors of prostaglandin synthesis was compared with the activity of two combinations of classical radical scavengers. Since the efficiency of both groups of compounds was similar, the role of free radicals in viral reactivation was strongly assumed, whereas the role of prostaglandins has to be revisited. All drugs tested were effective in the early phase after explantation of the latently infected ganglia and could be omitted in the later stages of co-cultivation. As free radicals are suspected of activating nuclear factors, it was of interest to confirm the enhancement of HSV reactivation by DNA hypomethylating drugs. 5-Azazytidine proved to be more effective than dimethylsulfoxide (DMSO). The effect of combinations of DMSO with inhibitors of prostaglandin synthesis was dependent on the mode of action of the inhibitors.